1. Field of the Invention
The present invention relates generally to a system for wiping a windshield and more particularly to mechanisms for mounting a wiper blade with respect to a windshield.
2. Description of the Related Art
A typical system for wiping a windshield includes a wiper blade assembly and an arm assembly. The arm assembly is typically mounted to a pivot shaft which is reciprocally rotated by a mechanical linkage connected to a wiper motor. The resulting reciprocal rotation of the arm assembly causes the wiper blade assembly to wipe across the windshield. A second arm assembly with a second wiper blade assembly is frequently provided as well.
A wiper blade assembly typically includes a rubber wiper blade 20, a typical end view of which is shown in FIG. 1. Wiper blade 20 comprises a wiping portion 22, a base 24, and a hinge 26.
Wiper blade 20 is shown in wiping position against a windshield 16 in FIG. 2. For the purposes of this discussion, assume that wiper blade 20 is moving in the direction depicted by arrow 28 in FIG. 2. Because wiper blade 20 reciprocates across windshield 16, wiper blade 20 will also at times move in the direction opposite to direction 28. For minimization of wiper blade "chatter" and to provide advantages in wiping efficiency, it is desirable for wiping portion 22 to assume a positive "attack angle" with windshield 16 when wiper blade 20 is wiping windshield 16. The "attack angle" is denoted as angle .alpha. in FIG. 2. The attack angle is the angle of inclination of the axis 30 of wiping portion 22 with respect to a normal 31 to the surface of windshield 16. Attack angle .alpha. is considered to be positive when wiping portion 22 is tipped away from the direction of motion (here, 28) of wiper blade 20, as in FIG. 2.
The typical wiper blade 20 in FIG. 2 is able to assume a positive attack angle due to hinge 26. As wiper blade 20 moves in direction 28, hinge 26 allows wiping portion 22 of wiper blade 20 to rotate to a position defining a positive attack angle. When wiper blade 20 reverses direction, hinge 26 allows wiping portion 22 to rotate such that a positive attack angle is re-established, despite the reversal of direction of wiper blade 20.
Although generally effective at maintaining a positive attack angle, a wiper blade employing hinge 26 has at least two disadvantages. First, when wiping portion 22 rotates during reversal of travel of wiper blade 20, a corner 32 or 34 of wiping portion 22 comes into contact with base 24 of wiper blade 20. Because rotation of wiping portion 22 generally occurs quickly and with significant force, the contacting of corner 32 or 34 with base 24 causes considerable noise. This noise can be annoying to persons inside the vehicle. The second disadvantage of a wiper blade employing hinge 26 is manifested during situations in which there is a particularly low coefficient of friction between wiping portion 22 and windshield 16. In such a case, the low friction can be insufficient to facilitate rotation of wiping portion 22 when direction of travel of wiper blade 20 reverses. Wiping portion 22 thus assumes a negative attack angle, which can cause significant chatter against windshield 16 and a considerable reduction in windshield wiping efficiency.
U.S. Pat. No. 4,800,610 discloses a windshield wiper with a gear mechanism to maintain a constant attack angle despite varying curvature of a windshield. However, this invention does not provide reversal of the attack angle between the wiper blade and the windshield. If such reversal is desired, other means must be employed to produce the reversal.
Therefore, a windshield wiping system which provides attack angle reversal in a relatively quiet and reliable manner will provide advantages over the prior art.